To provide an after-treatment device of an image forming apparatus, which after-treatment device binds recorded sheets of paper by stacking them, and stapling them at their middle portion, and then folding back them at their middle portion, and can improve an appearance of the bundle of folded-back sheets of paper by accurately arranging its edges on the opening side.

The after-treatment device of the image forming apparatus comprises a staple tray 25 which receives and stacks the recorded sheets of paper discharged from the image forming apparatus, a middle stapling staple 30 for stapling the middle portion of the stacked bundle P of the sheets of paper arranged on the staple tray 25, a middle folding-back unit 45 for folding back the bundle P of the sheets of paper stapled at their middle portion, and further a cutting means 50 for accurately trimming the opening side edges of the bundle P of the sheets of paper folded back by the middle folding-back unit 45.